The present invention relates in general to models employed in restorative or prothetic dentistry, and more particularly to an indexing and locating pin fixed in a removable die of a model and received by a base to which the model is secured.
In the making of a fixed dental prothesis (i.e. gold and ceramic crown or bridge) requires that an accurate replica of the existing teeth and gums (dentition) be made. The replicas, herein referred to as models, are made by taking an impression of dentition. Material capable of adapting to the exact shape and dimension of the dentition are, for example, silicon rubber and agar hydrocolloid. The impression is a "negative" model in which plaster, such as calcium sulfatehemihydrate, is poured. Upon removal from the impression, the plaster forms the model on which the prothesis is constructed. The model is secured to a base of plaster to increase its overall strength.
The plaster model comprises teeth which are replicas of the teeth remaining in the mouth of the patient and a replica of the teeth to be prepared by prothetic or restorative dentistry. It is the teeth prepared for the prothesis to be developed upon that employs an indexing and locating pin.
The replica of the teeth to be prepared by prothetic or restorative dentistry is known as a die. It is desired in restorative dentistry that the die be removable from and replaceable in the model. Further, the die should be indexed in the model to reposition itself in all planes after removal. The replaced die should not vary or move from its original position in the model in any plane more than .+-.0.001 inch. Also the die in its vertical removal movement from the model should be free to move in all planes immediately upon the vertical removal action to provide free-play for removal of undercut or eccentric dies. The removal is in a direction perpendicular to the occular plane. Of course, the removal is from the base upon which the model is secured. While disposed in the model, the die should be retained in place as to be unyielding to the normal forces and pressures, for example torsion, that may be applied to it during the fabrication of the prothesis. Thus, a die must be capable of being indexed in the model, must be removable with facility and must be stable against forces applied thereto during the fabrication of the prothesis.
Heretofore, dowel pins have been employed for indexing a removable die of a model. Such dowel pins have generally been made of brass. The dowel pins were fixed in the die and extended therefrom to be received by a suitable opening in the base. The dowel pins serve to index and locate the die relative to the model. Typically, the dowel pin had a cylindrical configuration or a surface equi-distance from the axis of the pin. At times, a portion of the cylindrical surface would have a flat taper portion extending in the axial direction equi-distance from the axis thereof.
Customarily, the dowel pin had a knurled end for retention in the die. In some instances, adhesive, such as cyano acrylate ester, on a smooth surface was employed to secure the dowel pin in the die. At other times, the dowel pin had a flared or flanged end for retention in the die.
Additionally, the dies heretofore employed had two separate and independent dowel pins extending from the die. Some dowel pins used plastic sleeves around the indexing portion of the pin for greater accuracy in indexing and locating the die in the model. The sleeve additionally may have facilitated the removal of the die from the model.
In the patent to Weissman, U.S. Pat. No. 3,153,283, there is shown a dowel pin with a complementary dowel sleeve, which are cast from metal. The dowel pin has a tapered shank at the lower portion thereof. The sleeve afforded indexing for the pin removable therefrom and insertable therein. Additionally, the sleeve facilitated the insertion and removal of the pin.
An Accu-Twin pin sold by Unitek Corporation of Monrovia, California, employed an indexing leg and a locating leg joined by a rectilinear portion. However, the surfaces of the legs are equi-distance from the axis of the legs, respectively, such as one would find in a cylindrical surface, or flat surfaces parallel to the longitudinal center line. A Pin-Dex pin manufactured by Whaldent, employed two separate and independent pins as above mentioned.
In the patent to Stengle, U.S. Pat. No. 3,478,428, a row of parallel, conically shaped guide pins were fixed to a retaining base in apparatus to facilitate the production of cast dental models. The pins serve as guide members for the die for indexing the same in the model. At least one pin in imbedded in each die. One surface of each pin is flattened to prevent rotational movement of the die.
The patent to Waltke U.S. Pat. No. 3,453,736 discloses dowel pins of tubular and conical shapes. Additionally, a dowel with the lower end thereof conical and the upper end thereof flared is also shown therein.
Other patents of interest are:
U.s. pat. No. 2,705,837 PA1 U.s. pat. No. 2,655,724 PA1 U.s. pat. No. 1,867,300 PA1 U.s. pat. No. 1,780,117
The pins heretofore employed in prothetic or restorative dentistry for removably mounting the die in the model exhibited poor indexing. The pins using flat surfaces had a tendency to wear the index surfaces into the plaster. The dies are subject to high torsional forces during the fabrication of the prothesis and thus the wearing of the index surfaces caused position and rotational error. The pins using knurls, flanges, or flares tend to be easily loosened from the die to result in poor retention. This occurs because of shallow insertion into the die or because of soft plaster used in the fabrication of the die. Pins using adhesive tend to be unreliable because of poor adhesion caused by moisture retained in the plaster.
Tapered pins with one or more flat surfaces parallel to the longitudinal center line of the pin restrict movement in the direction adjacent to the flat surface. Cylindrical pins are limited in movement to a direction perpendicular to the occlusal plane.